A complex sum method of quantifying susceptibilities in cylindrical objects: the first step toward quantitative diagnosis of small objects in MRI.

A complex sum method of quantifying the magnetic susceptibility of a long, narrow cylinder embedded in a uniform medium has been developed. The radius of the cylinder can be as small as one pixel. The susceptibility inside the object is extracted from the magnetic resonance complex images, using two concentric circles around the axis of the cylinder. The numerical simulations of this complex sum method are in good agreement with the phantom studies. Specifically, the method was tested with a susceptibility difference of -9 ppm to mimic air/tissue interface in the human body at 1.5 T with an echo time of 5 ms. Phantom studies using an air-filled cylinder in a solidified gel have shown that the susceptibility of the gel cannot be determined by the usual least-squares-fit method but can be determined by the complex sum method to within 5-10% of the expected value.